.. just for the sake of completeness, if you want to be more selective ..
(setq ss1 (ssget "_X" '((0 . "INSERT") (2 . "*SHEET,*SHEET01"))))
(setq ss2 (ssget "_X" '((0 . "INSERT") (2 . "x_SHEET,y_SHEET,main_SHEET,SHEET01"))))
.. note that there are no spaces after the comma